1. Field of the Invention
This invention relates to new and improved means for sensing temperature and, more particularly, to a temperature sensor generating a digital output by applying a thermally induced mechanical stress to a force-sensitive resonator.
2. Description of the Prior Art
Temperature is a basic physical parameter that cannot be measured directly. It can only be measured indirectly by measuring a physical change caused by a temperature change. The main methods for measuring temperature are measuring the change in density of a gas with temperature (gas thermometer), measuring the change in electrical property of an object with temperature (e.g., the platinum resistance thermometer and the thermocouple), and measuring the difference in thermal expansion coefficients of substances (e.g., the liquid thermometer and bimetallic strip thermometer). None of these previous measurement devices are inherently digital.
The only inherently digital measurements available today, as described hereinafter, all suffer from one problem or another. Commerically available digital temperature sensors utilize a quartz resonator having a temperature-induced frequency change occurring because of inherent properties of the quartz.
A number of inherently analog output thermometers may be coupled to analog-to-digital convertors in order to produce digital temperature sensors. Such devices as thermocouples with voltage-to-frequency convertors and oscillator circuits using temperature-sensitive capacitive or resistive elements can yield a digital-type output. In general, these sensors suffer from poor accuracy and stability, excessive complexity, insufficient reliability, and relatively high power consumption.
Direct digital outputs may be obtained by measuring the frequency of a quartz crystal or tuning fork whose output is a function of temperature. These units have a relatively low sensitivity to temperature and are expensive, both to fabricate and instrument.
U.S. Pat. No. 2,456,811, issued to Blackburn, discloses a temperature measuring system in which piezoelectric crystals oscillate at frequencies dependent upon temperature.
U.S. Pat. No. 2,732,748, issued to Grib, describes a temperature compensation technique for tuning forks using bimetallic elements.
U.S. Pat. No. 3,553,602, issued to Brothers et al., discloses a crystal oscillator temperature-sensing system which determines the temperature on one crystal surface with respect to a reference temperature on the other crystal surface by measuring the operating frequency of the oscillator.
U.S. Pat. No. 3,950,987, issued to Slezinger et al., discloses a piezo-optic measuring transducer in which the difference in thermal expansion of the crystals is measured by detection of the crystals' optical properties, not their resonant frequencies.
U.S. Pat. No. 4,039,969, issued to Martin, discloses a quartz thermometer using a single crystal constructed to have two separate oscillation sections. One section oscillates at a standard frequency, whereas the other section oscillates at a frequency dependent upon the temperature. The two frequencies are compared to determine the temperature.
The prior art devices as described in the above patents cannot meet the desired objectives of an inherently digital-type output, high sensitivity, excellent accuracy and stability, low power consumption, small size and weight, fast response time, high reliability, and low cost.
In an unstressed condition, under constant environmental conditions, a load-sensitive resonator has a unique resonant frequency determined by its dimensions and material composition. The resonant frequency of a flexurally vibrating resonator increases under tensile loading and decreases under compressive loading. A number of load-sensitive transducers utilizing this principle have been developed.
Force-sensitive crystals in which loads are applied to the crystals near the nodal points are described in U.S. Pat. No. 2,984,111, issued to Kritz, and U.S. Pat. No. 3,093,760, issued to Tarasevich.
U.S. Pat. No. 3,470,400, issued to Weisbord, describes a single-beam force transducer with an integral mounting system which effectively decouples the beam vibrations from the mounting points through a spring and mass arrangement.
U.S. Pat. No. 3,238,789, issued to Erdley, discloses two tines or bars vibrating 180 degrees out of phase such that the reactive forces and moments cancel.